This invention pertains generally to polarization control in fiber optic and free-space optical systems. Specifically the present invention relates to an optic apparatus for depolarizing polarized light in optical fibers and in free space.
Many components in advanced fiber optic communication (T. Okoshi, xe2x80x9cRecent Advances in Coherent Optical Fiber Communication Systems,xe2x80x9d J. Lightwave Technology, Vol. LT-5, No. 1, pp. 44-52, 1987) and sensing systems, such as interferometers and electro-optic modulators, are polarization sensitive. In order for these polarization sensitive devices to function properly, an input light""s polarization state is precisely aligned with a particular axis of the devices. Unfortunately, the polarization state of light propagating in a length of standard circular fiber varies along the fiber due to random birefringnece induced by thermal stress, mechanical stress, and irregularities of the fiber core (I. P. Kaminov, xe2x80x9cPolarization in Optical Fibers,xe2x80x9d IEEEE J. Quantum Electronics, Vol. QE-17, No. 1, pp. 1-22, 1981). Thus typically, a standard optic fiber outputs elliptically polarized light with varying degrees of ellipticity, and with the major elliptical axis at an arbitrary angle relative to a reference orientation.
One prior art method of solving the polarization problem utilizes polarization controllers. Polarization controllers, including tri-plate controllers, fiber tri-loop controllers, and the Yao controller (see Photonics Spectra, April issue, pp. xxx, 1998), typically convert an arbitrary polarization state into a desired polarization state. However, such polarization controllers cannot accommodate rapid polarization fluctuations in the fiber and therefore are unsuitable in systems where the polarization state fluctuates due to time dependent thermal or mechanical stresses on the fiber, or due to polarization fluctuations of the laser light itself.
Another method of solving the polarization fluctuation problem is to depolarize polarized light. One prior art m method of depolarizing light utilizes an electro-optic modulator to rapidly modulate the polarization state. However, the electro-optic modulator output is not truly depolarized. The output of the electro-optic modulator depolarizer only appears depolarized to an observer or detector having a response slower than the modulation speed. Another disadvantage of the electro-optic modulator depolarizer is high cost. Typical electro-optic modulator systems utilize a high frequency microwave signal source and an expensive electro-optic modulator. A third disadvantage of such an electro-optic depolarizer is the high loss, resulting from coupling between optical fibers and the waveguide in the electro-optical modulator.
A second prior art method for depolarizing light uses a recirculating fiber loop which includes a 2xc3x972 fiber coupler with two input ports, 1 and 2, and two output ports, 3 and 4. (xe2x80x9cTunable single mode fiber depolarizerxe2x80x9d by P. Shen, J. C, Palais, and C. Lin, Electronics Letters, Vol. 33, No. 12, pp. 1077-1078). The output port 4 is connected with input port 2 to form a recirculating loop. A first polarization controller is placed at input port 1 and a second polarization controller is placed inside the loop. The loop length is much larger than the coherence length of the input light so that the recirculating beams do not interfere with one another. Due to the incoherent addition of the recirculating beams, the output at port 3 is the superposition of different polarization states with different intensities. Depolarization occurs by averaging over the many different polarization states of the recirculating beams. The degree of polarization at output port 3 depends on the input state of polarization, the coupling ratio of the coupler, and the birefringnece of the fiber loop. By properly adjusting the two polarization controllers, polarized light entering input port 1 exits output port 3 unpolarized.
One disadvantage of using a recirculating fiber loop is that the time coherence of the depolarized beam is degraded and therefore may not be suitable for coherent communication systems. In addition, interference noise may arise when the loop length is not sufficiently longer than the coherence length of the input beam. Furthermore, device performance depends strongly on the input signal""s coherence length and thus the recirculating fiber loop is not suitable for systems where diversified signal sources are present. Finally, the device may be very bulky due to the long loop length (as long as a few km for DFB lasers).
Due to the disadvantages of prior art methods of depolarizing light described above, improved method of depolarizing light is needed.
The present invention relates to a method and apparatus of depolarizing or randomizing polarization states of an optical beam. One embodiment of the invention uses a wedged birefringent crystal with its wedge formed by many small steps. Another embodiment of the invention contains many randomly oriented birefringent crystal chips. Different parts of the optical beam passing through the apparatus experience different retardations and exit with different polarization states, resulting a spatially depolarized light beam. Focusing the spatially depolarized beam into an optical fiber results in a depolarized guided wave.